1. Field of the Invention
This invention relates to the field of microfluidics. More particularly, this invention relates to photolithographic methods for fabricating microfluidic devices having embedded metal conductors.
2. Description of Related Art
There has been tremendous growth over the past 5 years in the use of micromachining for fabricating microstructures, microsensors, and microfluidic devices, and in integrating these microstructures with electronic circuits. Micromachining is the process of forming structures having micron-sized detail by producing patterns in layers of material deposited on a substrate. The material layers can be formed using a variety of processes, including vapor deposition and spin coating. Patterns are produced in these material layers by processes such as precision physical machining, chemical etching, laser ablation, focused ion beam etching, ultrasonic drilling and electrodischarge machining, to yield the micromachined device.
Concurrently, uses for microfluidic devices are becoming increasingly widespread. Microfluidic devices are used in applications ranging from biological assays (the so-called xe2x80x9clab-on-a-chipxe2x80x9d devices) to drug delivery systems to manufacturing processes for pharmaceuticals and cellular macromolecules. Microfluidic devices can even be used to transmit force and energy in hydraulic systems. For example, microfluidic devices may be used in the design of smart surgical tools where the motion of the human hand must be scaled down to sub-millimeter dimensions, with a corresponding reduction in force. Microscale devices also permit the assembly of a multiplicity of different effector devices in one compact, interconnected system. For example, individual microfluidic accessories such as mixers, micro-contactors, reactors, pumps, and valves may be added on a substrate containing microfluidic channels that connect such components in a microfluidic device.
Microstructure technology offers distinct advantages over xe2x80x9cmacroscalexe2x80x9d technology, including, for example, the ability to perform efficient and rapid chemical analyses at a lower cost per analysis, because of decreased sample volume requirements and increased throughput. Small sample volumes are advantageous because they allow a user to perform multiple analyses in parallel using a single sample on a single chip. Smaller sample volumes are also advantageous in instances where the amount of material is limiting, such as in isolating rare natural products or in neonatal care where the amount of blood drawn from sick infants is preferably kept to a minimum.
A variety of microfluidic applications require electrical conductors. Conductors are used to form electrical interconnections (xe2x80x9cinterconnectsxe2x80x9d) between elements of a microfluidic device, such as electrodes, and elements external to the device, such as power sources. Such interconnects can provide electrical flow to the electrode to power electrohydrodynamic pumps or to induce electrolysis of a sample fluid. Conductors may also be used as resistive heaters for sample fluids or as temperature sensors in microfluidic applications.
Developments in the semiconductor processing industry have facilitated the fabrication of micron-sized structures, including sensors and monitoring systems that can be used in microfluidic devices. The fabrication of microfluidic devices requires a method of producing fluidic connections, referred to as microchannels, and electrical interconnects between regions of a single device or between a device and accessories such as automatic valves, pumps, or syringes. Existing methods of forming microchannels and electrical interconnects involve a complex process having multiple steps including via-filling, sealing, firing, lapping and polishing. These processes are time-consuming and costly and have questionable reliability. Low-cost methods such as photolithography have been used extensively in the fabrication of integrated circuits that require numerous electrical interconnections, but have not been applied in the field of microfluidic devices. Additionally, existing methods of fabricating microfluidic devices are not useful in forming interconnects having dimensions of less than 0.5 mm.
U.S. Pat. No. 5,457,073 to Ouellet and U.S. Pat. No. 5,905,307 to Onoda disclose methods for manufacturing semiconductor wafers by using photolithography and etching to create contact holes connecting two metallization layers separated by an electrically-insulating sublayer.
U.S. Pat. No. 5,846,860 to Shih et al. discloses a method of using tetraethylorthosilicate (xe2x80x9cTEOSxe2x80x9d) to form improved embedded contact junctions on semiconductor wafer.
Thus there remains a need in the art for simple, low cost, photolithographic methods for fabricating electrical interconnects and fluid-handling elements in microfluidic devices having dimensions, where the interconnects and fluid-handling elements have widths and depths less than 0.5 mm.
The invention disclosed herein provides microfluidic devices having embedded metal conductors and methods for manufacturing such devices. The present invention solves the above-referenced problems in the art by utilizing photolithographic methods for fabricating fluidic pathways and electronic interconnects in microfluidic devices, thereby reducing the cost and complexity of the fabrication process in comparison to traditional methods using via-filling, sealing, firing, lapping and polishing.
In a first aspect, the invention provides low-cost photolithographic methods for fabricating microfluidic devices comprising an electronic component having a substantially flat, planar surface and conductors embedded therein, and a fluid-handling component comprising a contoured surface affixed to the electronic component, wherein the contoured surface forms one or a plurality of cavities between the electronic component and the fluid-handling component. The electronic component preferably comprises a substrate having a surface, a patterned layer of electrically-conductive metal deposited on a portion of the surface of said substrate, and a layer of electrically-insulating material deposited on the patterned electrically-conductive metal layer and on the portion of the substrate surface not covered by the patterned electrically-conductive metal layer. The invention utilizes insulating materials that have good conformal or planarizing properties, wherein the conformal or planarizing properties of the insulating material result in a substantially flat, planar surface for the electronic component after depositing the insulating material on the substrate. Flat, planar surfaces are particularly desirable as they facilitate affixing the fluid-handling component to the electronic component.
The patterned layer of electrically-conductive material preferably functions as an electrical connection between an electrical source and an electrode extending into one or a plurality of cavities formed between the electronic component and the fluid-handling component, or as a heater for the contents of the cavity formed between the electronic component and the fluid-handling component. The cavities in the fluid-handling component preferably comprise a pattern of fluid-handling elements, or microchannels, or alternatively comprises a reaction chamber.
In a preferred embodiment, the invention provides a microfluidic device for performing electric-field lysis. In this embodiment, the contoured surface of the fluid-handling component forms a pattern of microchannels between the fluid-handling component and the electronic component. The electronic component in this embodiment additionally comprises an electrode extending from the embedded conductor into the pattern of microchannels, wherein the electrode introduces current into a sample fluid containing the cells to be lysed.
In an alternative preferred embodiment, the invention provides a microfluidic device for performing the polymerase chain reaction. In this embodiment, the contoured surface of the fluid-handling component forms a reaction chamber between the fluid-handling component and the electronic component. The embedded conductors of this embodiment are used as resistive heaters for a sample fluid contained in the reaction chamber and are separated from the contents of the reaction chamber by a series of electrically-insulating layers.
In a second aspect, the invention provides methods for using photolithography foil fabricating embedded conductors, electrodes, microchannels and reaction chambers in electronic and fluid-handling components of the invention. Specifically, the invention provides methods for producing the electronic components of the microfluidic devices of the invention whereby a photomask is used to transfer a pattern defining an electronic or fluid-handling element onto a layer of photoresist deposited on the substrate by exposing the photoresist layer to UV light through the photomask. The photoresist is then developed leaving the deposition pattern for the conductor or electrode materials or for etching of fluid pathways. The invention also provides methods for producing the fluid-handling components of the microfluidic devices of the invention whereby the pattern defines a mold on which the fluid-handling component material is deposited to form the contoured surface of said fluid-handling component.
Specific preferred embodiments of the present invention will become evident from the following more detailed description of certain preferred embodiments and the claims.